DIY: Generating Ramp/Sawtooth from PWM

Lack of any good tutorial to produce a ramp/sawtooth wave from a PWM signal urged me to make one! This post was a part of one of my recent ongoing projects, i.e., Redesigning the Semiconductor CURVE TRACER, here at CEDT, NSIT.

I was to produce a sawtooth wave from a PWM signal using Arduino nano, and here are my observations:

Producing a ramp/sawtooth wave out of a PWM signal merely involves passing a PWM signal of varying duty cycle through a low pass filter.

Here’s the code I used :

---

/*Code for prodcing a Sawtooth wave from PWM*/
int pwm_pin = 9;          
int brightness = 0;  

int upramp(int brightness)
{
  while(1)
  {
  brightness+=5;
  delay(100);
  analogWrite(pwm_pin, brightness);
  if(brightness>250)
  {
  digitalWrite(pwm_pin,LOW); // Statement:1 for sudden drop in output HIGH ---> LOW
  brightness=0; 
  break;
  }
  }
  return brightness;
}

// the setup routine runs once when you press reset:
void setup() {
  // declare pin 9 to be an output:
  pinMode(pwm_pin, OUTPUT);
}

// the loop routine runs over and over again forever:
void loop() {
  brightness=upramp(brightness);
 }

This code is pretty straightforward; I am not going to explain it. (Do let me know in case of any query in the comment section below).
After our PWM signal is set up, we need to look for a decent Low Pass filter; I tested both passive RC filters and active sallen key filter.
First, let’s try the RC filter:

RC filter:

But, before leaving, just have a look at fall time in case of RC LPF and Sallen Key LPF. Also, the contrast between Vmax and Vmin in both cases.
Apart from the crisp output, Sallen Key can also be configured to provide gain to the output signal, thus providing an added advantage.
Do let me know in case of any query/feedback, would love to hear one!
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